Deep into the structure of the first galaxies: SERRA views

Abstract

We study the formation and evolution of a sample of Lyman Break Galaxies in\nthe Epoch of Reionization by using high-resolution ($\\sim 10 \\,{\\rm pc}$),\ncosmological zoom-in simulations part of the SERRA suite. In SERRA, we follow\nthe interstellar medium (ISM) thermo-chemical non-equilibrium evolution, and\nperform on-the-fly radiative transfer of the interstellar radiation field\n(ISRF). The simulation outputs are post-processed to compute the emission of\nfar infrared lines ([CII], [NII], and [OIII]). At $z=8$, the most massive\ngalaxy, `Freesia', has an age $t_\\star \\simeq 409\\,{\\rm Myr}$, stellar mass\n$M_{\\star} \\simeq 4.2\\times 10^9 {\\rm M}_{\\odot}$, and a star formation rate\n${\\rm SFR} \\simeq 11.5\\,{\\rm M}_{\\odot}{\\rm yr}^{-1}$, due to a recent burst.\nFreesia has two stellar components (A and B) separated by $\\simeq 2.5\\, {\\rm\nkpc}$; other 11 galaxies are found within $56.9 \\pm 21.6 \\, {\\rm kpc}$. The\nmean ISRF in the Habing band is $G = 7.9\\, G_0$ and is spatially uniform; in\ncontrast, the ionisation parameter is $U = 2^{+20}_{-2} \\times 10^{-3}$, and\nhas a patchy distribution peaked at the location of star-forming sites. The\nresulting ionising escape fraction from Freesia is $f_{\\rm esc}\\simeq 2\\%$.\nWhile [CII] emission is extended (radius 1.54 kpc), [OIII] is concentrated in\nFreesia-A (0.85 kpc), where the ratio $\\Sigma_{\\rm [OIII]}/\\Sigma_{\\rm [CII]}\n\\simeq 10$. As many high-$z$ galaxies, Freesia lies below the local [CII]-SFR\nrelation. We show that this is the general consequence of a starburst phase\n(pushing the galaxy above the Kennicutt-Schmidt relation) which\ndisrupts/photodissociates the emitting molecular clouds around star-forming\nsites. Metallicity has a sub-dominant impact on the amplitude of [CII]-SFR\ndeviations.\n